The extracorporeal circulation is widely used in open heart surgery. However, the extracorporeal circulation itself is non-physiologic and harmful for the organism and is often associated with pathophysiological disturbances in various organs that may become apparent months to years after surgery. Protein denaturation during open heart surgery has been observed and confirmed, but little data is presently available on the contribution of the various components in heart-lung bypass to this event. The purpose of this study is to define the specificity and course of alterations in blood proteins which take place during and immediately after cardiopulmonary bypass. Our long-term goal is to determine the consequences of protein denaturation on numerous cellular, organ and body functions. These investigations should facilitate development of a new means for improving oxygen delivery systems, for evaluating the safety of new materials, and for monitoring bypass patients.